Exhibits details 19
Status: 16 November 2015
The transmitter module to the Würzburg systems
We recently obtained a so-called 'Lokomotiv' module constituting the transmitter to the small Würzburg FuMG 62 and the Giant Würzburg FuMG 65
(Neglecting the exceptional Urechse SÜ 62d which was not operated widely, maybe here and there experimentally only)
Viewing its appearance; Lokomotiv is a quite well fitting 'Tarnname'
The text A B does tell that this device can be operated at both wave bands A and B, also known as Inseln (introduction initiated as a response to the British Bruneval Raid of February 1942; also known as 'Operation Biting'). Each band covered a small spectrum of a few MHz wide. On the last photo of this series, the oscillator tuning scale shows the spreading of bands A, B and C. (range C at about 480-490 MHz necessitate an additional oscillator-coil change, this extra 'loop' was carried inside the oscillator module)
5.9 points onto the LS 180 particular filament voltage. All such valves carried voltage indications on it - marking at what exact filament voltage should be adjusted; as to guarantee reliable (long life) operation. (instrument tolerance 0.5%)
The circuit in front constitutes a ¼ λ tuning provision in series to the filament/cathode (actually, being a (half) capacitive linked tuneable stub; not visible, but the valve filament-leads + the quite long rods inside the LS 180 envelope were also incorporated in the ¼ wave length tuning)
A ¼ λ system provides at a quarter wave length a low impedance and is enabling oscillation at full wave length. Under all other circumstances (neglecting 3/4 λ), the cathode circuit is constituting a high impedance, preventing any kind of unwanted oscillation.
Genuinely, this tiny compartment was kept covered, as it once had been tuned at the regular operational frequency. But A and B indicate that two different spectra can be operated (albeit, that the transmitter had to be demounted and especially adjusted on to it, causing quite long operational delays, according late Fritz Trenkle of estimated 3/4 of an hour). A second option was that at this very cathode circuit a so-called Hilfssender HS 62/65 signal could be injected (energy about 10 mW), as to force the transmitter-pulse becoming quasi coherent.
For it special provision in the housing module or case was implemented.
A particular part being cut out of the left-hand housing
A small loop - coupling the HS 62/65 just tuned slightly higher in frequency (up to + 100 kHz versus, say 560, MHz), forcing the stage to start-up at that frequency and then jumping to its actual resonance frequency (when enough positive feedback energy was existing). Providing a strange spectrum - but having also so-called coherent spectral-lines; which being used further down the receiver chain and finally creating an interference pattern on the CRT screen. Known as Laus (lice) due to its peculiar screen appearance. The lice indicated that a target being velocity modulated (tracking an aircraft). Most signal components not having this kind modulation (like window or chaff) will be converted into an 'video dc-off-set'; and not becoming (well) visual.
For technicians: An important reason as to adjust the coherent oscillator frequency at a higher frequency - was to prevent that its continuous signal is overloading the rather sensitive receiver system. Therefore, the off-set was chosen such, that the Hilfssender (coherent oscillator) operated just at the edge of the IF bandwidth (250 kHz); thus the max. off set would have been, say, 125 kHz. Its frequency off set caused (also) an interference (Schwebung) between the high power transmission signal and the off-set signal of the coherent oscillator, when the radar signal returns to the receiving system.
There existed also a more sophisticated technique called: Tastlaus (keyed Laus), but this wasn't operated widespread.
Just working on this section I do realise that Mike Dean provided recently a photo where we can see some of its application.
Shown is the outfit of a Giant Wurzburg cabin at Skorping DK
The HS 62/65 module is visible in front, being the light colour box with a tuning knob. But the most interesting information is found on the front panel of the regular Eidechse module in the rear, mounted up inside the cabinet next to the side wall of this room. When you look closely, you might recognise A + B and W (maybe with a line through it). W stood for Würzlaus and Laus equals 'lice'.
The coherent oscillator or so-called Hilfssender 62/65 was operated by whom who monitored the Laus (lice) signal appearance on the main range CRT screen (LB13/40). The same person had also the duty to report on the full radar range-captures.
Adjusting HS 62/65 correctly was a delicate job and not everybody had a feeling for it. It was not only frequency- but also the accompanied injection level that counted.
The signal cable between the coherence oscillator (Hilfssender 62/65) and the transmitter injection provision is not visible.
(IIISC269088 "US National Archives" courtesy Mike Dean)
Photo taken during Operation Post Mortem in late June and early July 1945 at Skorping DK.
Viewing better what the tuneable cathode stub is about
The two contacts on the right-hand side quite heavy because the filament current is quite considerable.
Viewing the 'Lokomotiv' module from the mounting side
The coaxial connector on the right-hand side is the transmitter output connection (70 Ω).
The quite heavy resistor is a filament series provision, by which means the particular filament voltage can be adjusted individually. In the case of this module it should be adjusted at 5.9 V.
The small mechanical connection down on the right-hand side, is a provision for transmitter frequency fine tuning; often as to match onto the receiver channel optimally, or to escape interference from other systems.
Viewing inside the tuning compartment
K1 is the series tuned antenna out-coupling circuit.
In my perception it is a neat construction.
C4 is the HF suppressor capacitor of the 8 kV anode voltage
Please notice - that the output signal being 'pick up' from the field surrounding the LS 180 anode.
The way of inductive energy pick up is clearly understandable.
C 2 is a differential trimmer controlling frequency tuning as well as providing controlled positive feed back, and to ensure proper oscillation
I would like to close this page with a photo showing how the TX module being once mounted.
Sender-Überlegerer SÜ 62c
The TX module is the one on the far right-hand side (front-cover plate being removed)
It has to be noticed though, that the TX currently inserted is not the one we have seen in detail previously.
In the meantime it has been replaced by the new device.
The module with the (partly) circular tuning scale was called Michael Überlagerer (LO to the RX). A + B + C is indicating that this module could cover the Wismar Inseln A + B + C.
On the far left-hand side we notice the genuine mixer module.
In this fashion the Eidechse - as its codename once was, was the backbone of most Würzburg systems up to the bitter end.
By Arthur O. Bauer